THE STRUCTURE AND GROWTH OF STEMS 169 



becomes connected with the cambium of its neighbors. Thus 

 there is formed an unbroken cylinder of cambium, which is 

 referred to as the cambium ring. In this way the bundles 

 become connected ; in some stems they are closely side 

 by side from the first, in others they are at first bridged by 

 the extensions of the cambium and gradually grow 

 together, or, as in many stems of annuals, they may 

 remain distinct. (Compare Figures 24 and 5p.) In any 

 case, a complete cylinder of cambium is soon formed. 

 Remembering this cylinder of cambium and its power to 

 produce new cells, it is not difficult to see where the second- 

 ary wood comes from. In each growing season the cam- 

 bium forms new cells. This is done by lengthwise divi- 

 sions of its old cells. Of the new cells thus formed, those 

 on the inner side of the cylinder enlarge and become wood 

 cells (xylem) ; those on the outer side become cells of the 

 phloem. The new layers of wood, thus formed each 

 season, increase the diameter of the stem, and the cam- 

 bium which formed them is compelled to adjust itself to 

 this constant increase within it. The cambium does this 

 by growth of its cells in length and by some crosswise 

 divisions. Growth on the outside of the cambium is not 

 so great as the growth within it. The cells of the phloem 

 are not so large as those of the xylem. With these facts 

 in mind we can see how it happens that trunks of trees are 

 composed almost wholly of wood ; we can understand how 

 it happens that the cambium lies quite near the surface, 

 just under what is called the inner bark. This inner bark 

 is composed of cortex and phloem. 



The outer bark is composed of tissue called cork, often 

 with dead cortex in addition. This cork is produced by 

 what is called the cork cambium, a layer of cells which 



